The present invention relates to a thermal coating method for applying a functional layer onto a workpiece surface, particularly a thermal coating method for applying a running surface coating to a cylinder running surface of a crankcase of an internal-combustion engine.
From the state of the art, methods are known for the coating of cylinder running surfaces of crankcases of internal-combustion engines. By means of coating the cylinder running paths, the stability of the pistons and of the crankcase is to be increased, and simultaneously the sliding resistance of the pistons in the cylinders is to be reduced. So-called PVD, plasma or other thermal coating methods are known, by which single-layer or multilayer systems can be produced which have layer thicknesses of between 1 μm and 5 μm or layer thicknesses of between 1 mm and 3 mm. Generally, it is endeavored to create layers which are highly compressed and have minimal pores in order to achieve wear and stability characteristics that are as uniform as possible and in order to minimize tension of material in the layer.
The production of such compactly constructed layers requires high expenditures. Normally, the work has to take place in a vacuum or in an inert-gas atmosphere in order to prevent a pre-solidification and the formation of oxides. After the coating of cylinder running surfaces, these surfaces usually still have to be refinished. This generally takes place by honing or grinding, which in turn has the result that the layers will then be very smooth and even. However, cylinder running surfaces should not be too smooth and even. For this reason, the ground or honed cylinder running surfaces are usually subsequently roughened or structured in a defined manner, for example, by laser honing, laser dotting, plateau honing or other methods which require relatively high expenditures.
It is an object of the invention to provide a thermal coating method which is particularly suitable as a coating method for applying a running surface coating to a cylinder running surface of a crankcase of an internal-combustion engine and which requires no, or comparatively low, finishing expenditures.
This and other objects are achieved according to the invention by a thermal coating method for applying a functional layer to a workpiece surface, particularly a thermal coating method for applying a running surface coating to a cylinder running surface of a crankcase of an internal-combustion engine. The method has the steps of: melting a coating material by use of a melting device; applying coating material droplets to the workpiece surface by way of a gas jet aimed at the workpiece surface, which gas jet blows coating material droplets from a melting location of the melting device onto the workpiece surface; wherein the coating material droplets are cooled or rapidly frozen during their transport from the melting location to the workpiece surface.
According to the invention, the running surface coating is produced of a coating material which is first melted by use of a melting device. The coating material may, for example, be a material containing iron, which is present as wire or powder. The coating material is melted in a continuous process. For this purpose, an electric arc may be generated, for example, between two or more wires consisting of coating material, which electric arc continuously melts off the wire ends. The molten coating material will then be transported by way of a gas jet to the workpiece surface to be coated. In this case, tiny “coating material droplets” are blown from the melting location, i.e. from the wire ends of the melting device, onto the workpiece surface. For the coating of cylinder running surfaces, the melting device may be constructed, for example, in the shape of a so-called “rotary lance”, which is rotatable in the circumferential direction of the cylinder running surface to be coated and is displaceable in the longitudinal direction of the cylinder. By rotating the rotary lance in the circumferential direction of the cylinder and by a superimposed axial displacement of the rotary lance in the longitudinal direction of the cylinder, the entire running surface of a cylinder can be uniformly coated. The gas jet can be a compressed-air jet or a nitrogen jet.
In accordance with the invention, the coating material droplets are cooled or rapidly frozen during their transport from the melting location to the workpiece surface. Thus, in contrast to the coating methods known from the state of the art, coating material droplets are used which are pre-solidified or solidified. The acceleration of the coating material droplets onto the workpiece surface to be coated can, for example, take place in a cold environment—cooling chamber—or by use of a gas jet pre-cooled in a defined manner.
The cooling chamber may have a temperature that is in the range of between −40° C. and +5° C. The cooling chamber preferably has a temperature of approximately −20° C.
The coating or functional layer therefore consists of a plurality of coating material droplets that are pre-solidified or solidified in flight; i.e. of a combination of liquid or viscous deformed drops (also called flat cakes or lamellae) and pre-solidified globulitic drops which, after a honing of the functional layer, have a depth of 0.5 mm or more and generate “round oil pockets”.
When pre-solidified coating material droplets are used, relatively thick, definedly porous, lamella-type coatings can be produced, which are very suitable for a use as cylinder running surfaces.
According to a further aspect of the invention, the coating material droplets are cooled to such an extent during their transport from the melting location to the workpiece surface that they solidify in an area close to the surface. In this case, an exterior droplet shell of the coating material droplets solidifies, whereas the coating material is still liquid in the droplet interior. The coating material droplets are cooled to such an extent that 2% to 15%, preferably approximately or exactly 10% of the volume of the coating material droplets pre-solidify during the “flying time” from the melting location to the workpiece surface.
Tests have shown that a temperature difference of at least 300 K should prevail between the “atmosphere” surrounding the coating material droplets and the liquidus temperature of the coating material. The “liquidus temperature” is the temperature at which liquid coating material starts to solidify when there is a falling below this temperature. The difference between the temperature of the molten coating material droplets at the melting location, i.e. at the wire ends, and the surrounding atmosphere, i.e. the temperature of the gas jet or the cooling chamber, should preferably amount to at least 300 K.
In order to achieve the desired pre-solidification of the areas of the coating material droplets close to the surface, the “flying time” of the coating material droplets, i.e. the time period which the coating material droplets require from the melting location to the workpiece surface, should amount to at least 0.2 seconds, preferably 0.5 seconds. Good results can be achieved when the distance between the melting location and the workpiece surface to be coated is in a range of between 5 mm and 200 mm. In tests, a distance of approximately 4 cm was found to be advantageous. When coating material droplets are used that are pre-solidified in this manner, they are deformed and aligned in rows when impacting on the workpiece surface as a result of their kinetic energy, however, without fusing with the adjacent droplets. This results in a layer that is traversed by fine lamellae and fissures of a defined size and dimension.
It is an advantage of such an “amorphous” coating that, during a subsequent surface treatment of the coating, a defined number of pre-solidified drops separates from the layer, which results in a surface topography having a plurality of undercuts and relatively “deep” oil pockets (depth of oil pockets greater than 0.5 mm), which is advantageous for use as a running surface for a piston of an internal-combustion engine. The reason is that lubricating oil may accumulate in the undercuts or oil pockets. During the operation of the internal-combustion engine, this will then result in the formation of a uniform oil film between the piston rings and the cylinder running surface.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.